Process of making sodium aluminate



Patented May 23, 1939 UNITED STATES PROCESS OF MAKING SODIUM ALUMIINATE Richard Lloyd Davies, Philadelphia, Pa., assignor to Pennsylvania Salt Manufacturing Company, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Application August 18, 1937, Serial No. 159,795

11 Claims.

The present invention relates to an improved method for the preparation of a sodium aluminate product, and more particularly it relates to an economical process for the preparation of such products in a desired form.

l5 and of alumina hydrate, which process eliminates the application of heat during the reaction due to the sensible heat of the caustic soda employed and the heat of reaction, and simplifies the production of a technically anhydrous sodium alu- 20 minate product.

Other objects, including the provision of a method for the production of completely watersoluble sodium aluminate products in granular form from a cheap source of caustic soda, will be 25 apparent from a consideration of the specification and claims.

The processes generally used in the preparation of sodium aluminate are cumbersome, and timeconsuming, and involve costly manipulative oper- 30 ations. In the Bayer process, bauxite is digested with a solution of caustic soda, thus producing a solution of sodium aluminate which is difficultly separated from insoluble matter. The solution is concentrated, and the sodium aluminate is 35 recovered therefrom by crystallization, filtration,

etc. Another method involves the heating of bauxite or other aluminous material at a bright red heat with various materials, for example, sodium sulphate and carbonaceous material, or 40 sodium carbonate. The product thus produced contains insoluble material from 5% to and I the process involves expensive equipment, and high temperatures accompanied by high repair and fuel charges.

. An improved process of making sodium alumi- 5 stantially anhydrous product. Since the heat during the initial reaction is relatively low, little, if any, water formed by the reaction is removed during that step, and the subsequent heating step must, therefore, be relied upon to remove the water associated with the product. 5 The process of the present invention departs markedly from the known processes. Aqueous digestion is not employed, and while the reaction is between caustic soda and subdivided alumina hydrate, a relatively cheap form of caustic soda 10 is utilized. The caustic soda employed contains sufficient sensible heat when brought in contact with the alumina hydrate to cause the reaction to become complete without the application of additional heat, thus a material saving is provided. Furthermore, the caustic soda possesses sufficient sensible heat to remove at least a portion of the water from the initial reaction product to form a plastic mass which is solidified. The removal of a portion of the water also simplifies or eliminates the subsequent dehydration step. The term sodium aluminate product includes products Where the molecular ratio of NazO to A1203 is at least equal to 1 to 1. Thus, the product obtained as the result of the reaction of 1 mol of NazO and 1 mol of A1203 is known as sodium meta-aluminate, and the one obtained by reacting 3 mols of NazO and 1 mol of A1203 has the oxides present in the molecular ratio corresponding to sodium ortho-aluminate. The reaction for the formation of sodium meta-aluminate may be represented by equation (a) and ofsodium ortho-aluminate by equation (b):

If products are prepared using between 1 mol of NazO and 3 mols of NazO to 1 mol of A1203, intermediate products are obtained. Products containing a greater molecular ratio of NazO to A1203 than 3 to l are thought to be mixtures of an aluminate and caustic soda, and products of this type are of particular value in the detergent field. The molecular ratio of NazO to A1203 in the sodium aluminate product may, therefore, greatly exceed 3 NazO to 1, and the product may, for example, contain molecular ratios of NazO to A1203 of 5 to 1; 10 to 1; 15 to 1; 20 to l; and the like. As will hereinafter appear, when a completely soluble sodium aluminate product is desired, the molecular ratio of NazO to A1203 should not be less than 1.2 NazO to The term alumina hydrate as employed herein includes both the naturally occurring (bauxite) V and the artificially prepared alumina hydrates, and this reactant is used in subdivided form. Prepared aluminum trihydrate (Al(OH)3) as available on the market, such as the so-called Bayer alumina hydrate, or this product calcined at relatively low temperatures to produce a partially hydrated product are applicable for use. The advantage of the use of calcined alumina hydrate resides in the fact that the water-content of this reactant may thus be lowered. In fact, when alumina hydrate calcined at a temperature in the neighborhood of 600 C. is reacted with molten 100% caustic soda at temperatures above its melting point, for example, about 350 C. to 370 C. or higher in a molecular ratio of NazO to A1203 of 1.4 to 1 or higher, a technically anhydrous product is produced directly in the reaction vessel. In addition, more easily granulated products are obtained when the molecular ratio of NazO to A1203 is higher than 1 to 1, 1.2 to 1 and the like, for example, when the ratio of NazO to A1203 is 2 or 3 to 1, than is the case when aluminum trihydrate is reacted, although no difliculty with granulation is encountered when a large excess of caustic alkali above the ratio of the orthoaluminate is employed with aluminum trihydrate. There is a tendency for the alumina hydrate to become unreactive, and to present difiiculties clue to dusting losses during the reaction, if the calcination is conducted at high temperatures to remove a major portion of the water therefrom. For this reason, it is advantageous to carry out the calcination, if a calcined product is to be employed, at temperatures not in excess of about 600 C., and preferably at temperatures of 300 C. to 400 C. If the alumina hydrate is heated at 300 C., the water-content is reduced from 34% to 25.4%, and a further decrease in Watercontent results if the calcination temperature is higher.

Bauxite ores containing approximately 30% of combined water can also be used. For example, Arkansas, Georgia-Tennessee-Alabama, and South American (Dutch and British Guiana) bauxites may be utilized as thematerial to be reacted with the caustic soda. If a product of a high degree of purity is desired, aluminum trihydrate, or the calcined trihydrate is employed as the source of'the aluminate radical. If, on the other hand a cheaper grade of product is desired, containing some insoluble material, a powdered bauxite containing approximately 30% combined water may be used, as the percentage of sodium aluminate in the final product will depend at least in part upon the purity of the bauxite. Preferably, a bauxite is chosen which contains only a small percentage of silica, for example, not more than 6%, and, for this reason, the South American white bauxites are particularly applicable.

The molten caustic soda used in the reaction will contain about to NaOH. Obviously, to fulfill the requirement that molten caustic soda be employed, the caustic soda will be above its melting point, and, therefore possesses sufiicient sensible heat to bring about the complete reaction, without the aid of further heat, other than that generated exothermically by the reaction. The sensible heat of the molten caustic soda and the heat generated, generally will carry the reaction to completion in 2 to 5 minutes. In the event that a technically anhydrous, granular, product is to be prepared, the sensible heat of the liquid caustic soda is preferably sufiicient to remove a substantial portion of the water associated with the product. For this reason, if caustic soda containing between 95% and 100% NaOI-I is selected, the use of the product at its boiling point is recommended. As before stated, when 100% NaOl-I is used, the temperature is necessarily above its melting point (318 C.) and may advantageously be much higher, for example 400 C. to 600 C. The initial product containing the smallest amount of water will be obtained when 100 NaOI-I at a relatively high temperature is reacted with alumina hydrate calcined at a relatively low temperature. One of the economies of the process of the present invention resides in the fact that the molten caustic soda can be obtained directly from the concentrating equipment, for example, the caustic pots, and used in the process while it still contains the sensible heat imparted to it in its manufacture. Thus, molten caustic soda containing about 95% NaOH at its boiling point, or 100% NaOH at its finishing temperature, can be flowed directly to the reaction vessel where the process of the present invention is to be conducted.

The reactants are stirred during the reaction to insure contact between the molten caustic soda and the alumina hydrate, and the reaction is advantageously carried out, either as a continuous or an intermittent operation, in a simple device provided with a mixing arm or the like to provide the necessary stirring of the mass. The mixing may be continued until the plastic mass, first obtained is converted into a granular product in the mixer, or the reaction mass may be flowed or otherwise removed from the mixer in a plastic state, and thereafter converted into a solid state.

The products initially obtained, if hydrated as is usually the case, are of use in the trade without further treatment. If desired, a technically anhydrous product may be produced by subsequently heating the hydrated product. The dehydration is accomplished by placing the material in a drier which may take the form of a rotary kiln through which sufilcient hot air passes to dehydrate the material to form the anhydrous or substantially anhydrous sodium aluminate product. Complete dehydration at C. is relatively slow, but if a temperature of 250 C. or more is employed, the Water is removed to form a technically anhydrous product in a comparatively short time. Scrapid is this removal, that temperatures in excess of 250 C. have not been found necessary, although higher temperatures may be used; for example, 500 C. or in fact any temperature up to the melting point of the material heated. If the initial product in granular form is subjected to the secondary dehydration step, the technically anhydrous product is in the form of a granular, free-flowing product when removed from the drier. If an especially pure product is desired, the air used in the dehydrating step may be freed from its carbon dioxide.

The temperature of the particular molten caustic soda to be employed, as well as the type of alumina hydrate to be reacted therewith will depend on the properties desired in the sodium aluminate product obtained as the result of the reaction. If the presence of a small amount of insoluble material in the product is not objectionable, the choice. of materials to be reacted is of no particular moment. When, however, a totally water-soluble product is desired, more care in the selection of the reactants is required. Some difiiculty is encountered in producing a granular material from bauxite and liquid caustic soda of a concentration less than 100%, particularly where the ratio of NazOto A1203 is low, and, therefore, when bauxite is a reactant the use of molten caustic of 100% concentration is recom mended. Also, as previously stated, the use of molten 100% caustic and partially calcined alumina is advantageous in the production of granular products where the ratio of NazO to A1203 is above 1.2 to 1, for example, 2 or 3'Na2O 130 1 A1203.

It has been found that when a totally watersoluble product with as small a molecular ratio of NazO to A1203 as is feasible is the object of the process, a slight excess of caustic soda over the minimum theoretical ratio of 1 mol of NazO higher ratio are desired, the percentage of NazO is proportionately increased and the reaction carried out in the manner described. Thus, aluminum hydrate in amounts varying from 10 grams to 30 grams were added to 83.5 grams of molten caustic soda (100%) at 400 C. The reacting mass was stirred and as the product cooled, the mass granulated. In each case, homogeneous and satisfactory products were produced with a content of A1203 varying from 7% to 17%. The presence of the sodium aluminate in the caustic soda exerts a buffering action on the caustic soda, aiding it in its detergent properties.

All of the products, produced in accordance to 1 mol of A1203 should be used, for example, with the preceding examples, are hydrated, so-

1.20 mols of NazO to 1 mol of A1203 is the usual dium aluminate products of use in the trade. If minimum with a suitable adjustment of the retechnically anhydrous, granular products are deactants, if the temperature of the caustic soda sired from hydrated initial products, they are is sufliciently high to cause loss of alumina obtained by heating the initial products at suithydrate due to the dusting resulting from the able temperatures below their melting points to violence of the reaction. remove the water therefrom; for example, a tem- In typical examples, using a ratio of 1.2 mols perature of 250 C. to 500 C. may be employed. of NazO to 1 A1203, the latter being furnished by In the production of a technically anhydrous sub-divided aluminum trihydrate, the following sodium aluminate product directly in the mixing 5 results were obtained using molten caustic soda of device, 24.6 pounds of molten caustic soda concentrations of 100% and at temperatures from (100%) at 350 C. to 370 C. and 15.7 pounds of 318 C. to 600 C., the product being stirred in alumina trihydrate which had been heated at each instance until the reaction mass was 600 C. (10.7 pounds of the calcined product) granulated: were added to the mixer. The reaction was very Table Molecular Weight Caustic Weight Nmigtfiihoa 01:12:30 gg teigiugigra- 2111;111:2132 gggg Insoluble NazO A110:

Grams Percent C'. Grams Minutes Percent Percent Percent 1. 20 100 100 318 157 7 None 32. 18 43. 04 1.28 100 100 400 157 5 None as. 15 45. 0a 1. 100 100 600 157 4 None as. 55 47. 17

In the above table, the ratio of N20 to A1203 in vigorous and the stirring of the reacting mass the final product when molten caustic at 600 C. resulted in a granulated product which was comwas used rose to 1.35 due to the loss of some pletely soluble in water. alumina caused by the violence of the reaction Considerable modification is possible in the reat that temperature. In all the examples, the actants selected and the proportions thereof emmolten caustic soda possessed sufilcient sensible ployed, as well as in the steps of the process, heat to complete the reaction. It is evident from without departing from the essential features of the tabulation that the amount of water rethe invention. moved, as indicated by the difierence in the total I claim: percentages ofNazO and A1203, increased as the 1. The process of preparing a solid sodium sensible heat content of the caustic soda was inaluminate product which comprises separately creased. supplying heat to caustic soda containing from The use of sub-divided aluminum hydrate about 95% to 100% NaOI-I to provide a temperawhich has been calcined at 300 C. gave a product ture thereof above its melting point and admixcontaining even smaller amounts of water. The ing therewith a solid sub-divided alumina hycalcination of the hydrate at 300 C. lowered its drate, the reactants being present in such molecwater-content from 34% to 25.4%, and when this ular proportions that the NazO content is greater calcined product was reacted with molten caustic than the A1203 content, and the temperature of soda (100%) at 400 C. in the manner hereinthe caustic soda being above its liquefying point before described (1.2 mols of NazO to 1 mol of but below 600 C. to cause a reaction between A1203). a granular prod ning sodium said reactants and to free the mixture of at least aluminate 86.81%, and insoluble .97% was 0ba portion of the water to form a plastic mass, tained. without the application of further heat, during In the production of a product from South the reaction stirring the mixture, and converting American bauxite, 167 parts of powdered bauxite the reaction mass into a solid state.

Containing about 0% of A1203, 6% to 8% of 00m- 2. The process of preparing a solid, granular,

bined silica, titanium, and iron oxides, and 30% sodium aluminate product which comprises sepof combined water, were reacted with 100 parts arately supplying heat to caustic soda containing of molten caustic soda (100%) at 400 C. The from about 95% to 100% NaOH to raise it above reacting mass was stirred and the reaction started its melting point and admixin therewith solid in four minutes and was completed in about 10 sub-divided alumina hydrate, the reactants being minutes. The insoluble material in the bauxite present in such molecular proportions that the pp d i h s d a u t p u d, and NazO content is greater than the A1203 content, the total insoluble amounted to 8.29%. and the temperature of the caustic soda being In'the foregoing examples, a low ratio of NazO above its liquefying point but below 600 C. to

to 2 3 was p oyed. ut if p ucts having a cause a reaction between said reactants and to free the mixture'of at least suflicient water to permit granulation of the reaction product without the application of further heat, and during the reaction stirring the mixture to form a granular sodium aluminate product.

3. The process of claim 2 wherein the alumina hydrate is aluminum trihydrate.

4. The process of claim 1 wherein the alumina hydrate is aluminum trihydrate.

5. The process of claim 2 wherein the alumina hydrate is aluminum trihydrate calcined at a temperature below 600 -C.

6. The process of claim 1 wherein the alumina hydrate is aluminum trihydrate calcined at a temperature below 600 C.

'7. I'he process of claim 1 wherein the alumina hydrate is aluminum trihydrate calcined at a temperature below 600 (3., and wherein the molecular ratio of the NazO to A1203 in the reacting mixture is between 1.2 to 1 and 3 to 1.

8. The process of claim 1 wherein the alumina hydrate is bauxite, containing approximately 30% of combined water.

9. The process of claim 1 wherein the solid sodium aluminate product obtained is heated at a temperature below the melting point thereof to free it substantially of water and to form a substantially anhydrous sodium aluminate.

10. The process of claim 1 wherein the caustic soda is supplied directly from the concentrating equipment.

11. The process of claim 2 wherein the caustic soda is supplied directly from the concentrating equipment.

RICHARD LLOYD DAVIES. 

